The present invention relates to automated docking systems for space vehicles and, more particularly, the present invention relates to a video guidance sensor system for such docking system including a laser rangefinder for determining the range of a target vehicle relative to a chase vehicle.
Prior techniques used in determining the range between two spacecraft vehicles for automatic rendezvous and docking of such, includes vehicle radar, man in loop estimates, global positioning systems, lasers, loran, and video guidance sensor systems for processing optical images in determining range. The video guidance sensor system approach, which is of particular importance here, is based on the concept of using captured and processed images to determine the relative positions and attitudes of a video guidance sensor and target. However, conventional video guidance sensor systems tend to be bulky, heavy, slow and demand higher power requirements.
One prior video guidance sensor system uses two lights of predetermined wavelengths to illuminate a target. The target includes a pattern of filtered retroreflectors to reflect light. The filtered retroreflectors pass one wavelength of light and absorb the other. Two successive pictures or images are taken of the reflected light and the two images are then subtracted one from the other, thereby allowing for target spots to be easily tracked. However, due to its size, weight, power requirements and speed, the prior art video guidance sensor system is of limited use in applications requiring fast tracking of moving objects. Such a system is described, for example, in R. Howard, T. Bryan, M. Book, and J. Jackson, xe2x80x9cActive Sensor System for Automatic Rendezvous and Docking,xe2x80x9d SPIE Aerosense Conference, 1997, which is hereby incorporated by reference.
Another prior art video guidance sensor system uses a CMOS imaging chip and a digital signal processor (DSP) in order to provide higher-speed target tracking and higher-speed image processing. The faster tracking rates result in a more robust and flexible video guidance sensor. Because of these faster tracking rates, the video guidance sensor system can track faster moving objects or provide more data about slower moving objects. This video guidance sensor system is designed to be less complex, consume less power and volume and weigh less than previous systems. However, the video guidance sensor system is limited insofar as extended rangefinding and does not provide important backup capabilities nor an initial range estimate. Such a system is described, for example, in R. Howard, M. Book and T. Bryan, xe2x80x9cVideo-based sensor for tracking 3-dimensional targets,xe2x80x9d Atmospheric Propagation, Adaptive Systems, and Laser Radar Technology for Remote Sensing, SPIE Volume 4167, Europto Conference, September 2000, and in R. Howard, T. Bryan, and M. Book, xe2x80x9cThe Video Guidance Sensor: Space, Air, Ground and Sea,xe2x80x9d GNandC Conference, 2000, which are also hereby incorporated by reference.
In accordance with the present invention, a video guidance sensor system is provided for use in automated docking of a chase vehicle with a target vehicle, said system comprising: a passive target mounted on the target vehicle, said passive target including filtered retroreflectors for reflecting light received thereon; a video guidance sensor mounted on the chase vehicle, said video guidance sensor including: means for directing light of two predetermined wavelengths onto said filtered retroreflectors so that light of one wavelength reflected by said retroreflectors is received by said sensor; a camera for providing video images of the received light and producing a corresponding video output signal; a signal processing unit, connected to the camera, for receiving and processing said video output signal and for producing corresponding output signals; a computer for receiving said output signal from the signal processing unit, and for controlling operation of the chase vehicle based thereon so as to enable docking of the chase vehicle with the target vehicle; and a laser rangefinder, connected to said computer, for determining a range of the target vehicle relative to the chase vehicle and for supplying a corresponding range signal to said computer.
Preferably, the laser rangefinder includes a laser diode for producing laser light of the proper wavelength directed at said passive filtered target and an avalanche photodetector for receiving light reflected by said passive target and producing a corresponding output signal.
Advantageously, the laser rangefinder includes a diode laser pulse driver, electrically connected to said signal processing unit and to said laser diode, for providing a driver output signal for driving said laser diode.
Preferably, the laser rangefinder includes a wide-angle lens disposed in front of said laser diode for providing wide angle illumination of said passive target, a filter disposed in front of an avalanche photodetector, the filter being tuned to a predetermined wavelength of said laser diode, and a wide angle lens disposed between said filter and said avalanche photodetector.
Advantageously, the laser rangefinder includes an operational amplifier for receiving said output signal from said avalanche photodetector and for providing a corresponding amplified output signal.
Preferably, said diode laser pulse driver further produces an output control signal and said laser rangefinder includes a timing unit for receiving said amplified output signal and said output control signal and for, responsive thereto, supplying an output signal to said computer for use in determining the range of the target vehicle relative to the chase vehicle.
Advantageously, said rangefinder further includes a timing unit for measuring a time interval between production of a light pulse by said laser diode and detection of light by the avalanche photodetector, and for supplying a corresponding output signal to said computer for use in determining the range of the target vehicle relative to the chase vehicle.
Advantageously, the video guidance sensor system further comprises a filter disposed in front of the avalanche photodetector, said filter being tuned to a predetermined wavelength of said laser diode.
Advantageously, the video guidance sensor system further comprises a turning mirror for receiving light reflected by said retroreflectors.
Preferably, the signal processing unit comprises a digital signal processor and the computer comprises a single board computer.
Advantageously, the video guidance sensor system further comprises a power converter for supplying power to said video guidance sensor.
Preferably, the video guidance sensor system includes a solar filter disposed in front of said camera.
Advantageously, the video guidance sensor system further comprises a plurality of laser drivers coupled to said signal processing unit for driving said plurality of laser diodes so as to produce light.
Preferably, one of said laser diodes operates at a predetermined wavelength rejected by the retroreflector filter and a further one of said laser diodes operates at a predetermined wavelength accepted by the retroreflector filter.
Advantageously, the laser range finder comprises a computer, a modulator means for modulating the laser light produced by said laser diode and for producing a corresponding modulation frequency signal to said computer, an avalanche photodetector means for detecting said laser light and for producing a first phase signal, a phase detector for comparing said first phase signal and a further phase signal from said photodetector related to the light reflected by said passive target and for producing a corresponding relative phase signal based thereon, said computer determining the range of the target vehicle relative to the chase vehicle based on said modulation frequency signal and said relative phase signal.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.